In recent years, SC (Single Carrier)-FDMA (Frequency Division Multiple Access) scheme has achieved lower peak power than multi-carrier transmission schemes such as OFDM (Orthogonal Frequency Division Multiplexing) and can reduce power consumption. Therefore, much attention has been drawn to the SC-FDMA scheme. One of generation methods for generating SC-FDMA transmission signals is as follows. For each of unit time slots, M data symbols (where M is a natural number) are subjected to DFT (Discrete Fourier Transform), thereby converting the M data symbols into a frequency domain. Total N−M zeros (where N is a multiplier of 2, and is greater than M) are further inserted in both of high and low frequency sides. IFFT sampling at N points (corresponding to N/M-times oversampling) is thereafter carried out. Even after the N/M-times oversampling, Peak to Averaged Power Ratio (PAPR) is lower than that in the multi-carrier transmission methods. However, one of problems in the SC-FDMA is that the PAPR increases to be greater as compared with that before the N/M-times oversampling.
FFT Pre-processing method is one of methods for reducing the PAPR in the SC-FDMA scheme (e.g., see Non-Patent Document 1). According to the FFT Pre-processing method, X-times oversampling is performed before N/M-times oversampling which is performed when generating a transmission signal. The X-times oversampling is performed to select a data symbol which may cause a peak in the N/M-times oversampling. In this processing of selection, data symbol points are associated with signal points obtained by different operations of X-times oversampling, respectively, to determine whether each data symbol may cause a peak. Therefore, X needs to be 2 or greater (where X is a real number not smaller than 2). Further, oversampling rates of X and N/M should desirably be values close to each other in order to maximize effect of the FFT Pre-processing method. A conventional example will now be described below in a case of X=2 (N/M=1.7) where M=300 and N=512 are given.
A signal which may cause a great peak after N/M-times (1.7 times) oversampling is selected before the oversampling by using a result of 2-times oversampling. The amplitude of the selected signal is attenuated before DFT in the 1.7-times oversampling. In this manner, a high peak is prevented from occurring after the 1.7-times oversampling, which is a feature of this conventional example. A conventional FFT Pre-processing method will further be described below with reference to FIGS. 1 and 2.
A transmitter 1000 shown in FIG. 1 includes a data signal generation unit 1001, a DFT unit 1002, a 2-times point IDFT unit 1003, an amplitude reference attenuation signal selection unit 1004, a subtraction coefficient multiplication unit 1005, an N/M-times oversampling unit 1006.
In the transmitter 1000 shown in FIG. 1, where M data signals (M is a natural number) are included in each unit time slot, the data signal generation unit 1001 generates M data signals Din(M×v+1) to Din(M×v+M) (consecutive numbers from the first data in the first slot), in the with time slot (v is an integer not smaller than 0). The DFT unit 1002 is supplied with the data signals Din(M×v+1) to Din(M×v+M), performs DFT (Discrete Fourier Transform) at M points, and outputs DFT output signals Dout(M×v+1) to Dout(M×v+M).
The 2-times point IDFT unit 1003 is supplied with total 2M signals which are obtained by extrapolating total M zero-components from outsides of both ends of the DFT output signals, where the two ends correspond to high and low frequency components. The 2-times point IDFT unit 1003 generates 2-times oversampling signals by performing IDFT (Inverse Discrete Fourier Transform) at 2M points, and outputs the generated signals as 2-times oversampling signals Ddbl(2M×v+1) to Ddbl(2M×v+2M).
Referring to FIG. 2, the amplitude reference attenuation signal selection unit 1004 will now be described next. The amplitude reference attenuation signal selection unit 1004 includes an attenuation coefficient initial value generation unit 1101 and an amplitude reference attenuation coefficient calculation unit 1102.
The attenuation coefficient initial value generation unit 1101 generates and outputs attenuation coefficient initial values Y(M×v+1) to Y(M×v+M) which are all 1.
The amplitude reference attenuation coefficient calculation unit 1102 is supplied with the 2-times oversampling signals Ddbl(2M×v+1) to Ddbl(2M×v+2M) and the attenuation coefficient initial values Y(M×v+1) to Y(M×v+M). If the magnitude of a 2-times oversampling signal which is not at the same sampling time as the data signal before the oversampling exceeds a threshold C (C is a positive real number), then the attenuation coefficient initial value Y(M×v+g) (g is a natural number not greater than M) corresponding to a sampling time which is prior to the sampling time of the oversampling signal by one 2-times sampling time is changed to the resultant value expression 1 in FIG. 3 by the amplitude reference attenuation coefficient calculation unit 1102. The amplitude reference attenuation coefficient calculation unit 1102 outputs, as attenuation coefficients Wm(M×v+1) to Wm(M×v+M), results of changing the attenuation coefficient initial values Y(M×v+1) to Y(M×v+M). In the expression 1, y is a positive real number and abs( ) is a magnitude of the number in the parenthesis.
A result of N/M-times oversampling after attenuation coefficient multiplication can be set to a value close to a constant value (C) by setting an attenuation coefficient so as to decrease as the amplitude value of a 2-times oversampling signal Ddbl(2M×v+2g−1) increases. Since sizes of peaks after N/M-times oversampling depend on the attenuation coefficient, the attenuation coefficient should desirably be smaller in order to reduce PAPR. However, if the amplitude after multiplication of the attenuation coefficient is too small, there is a problem that reception characteristics deteriorate. Therefore, when the amplitude of the 2-times oversampling signal Ddbl(2M×v+2g−1) is small, the attenuation coefficient needs to be large.
The attenuation coefficient multiplication unit 1005 is supplied with the data signals Din(M×v+1) to Din(M×v+M) and the attenuation coefficients Wm(M×v+1) to Wm(M×v+M), and multiplies the data signals Din(M×v+1) to Din(M×v+M) respectively by the attenuation coefficients Wm(M×v+1) to Wm(M×v+M). The attenuation coefficient multiplication unit 1005 outputs multiplication results thereof as attenuation coefficient multiplication signals Sdin(M×v+1) to Sdin(M×v+M).
The N/M-times oversampling unit 1006 is supplied with the attenuation coefficient multiplication signals Sdin(M×v+1) to Sdin(M×v+M), and performs DFT at M points, thereby generating attenuation multiplication DFT output signals Sziout(M×v+1) to Sziout(M×v+M). The N/M-times oversampling unit 1006 extrapolates (N−M) zero signal points from outsides of both ends of high and lower frequency components of the attenuation multiplication DFT output signals Sziout(M×v+1) to Sziout(M×v+M) in order to obtain N points, where N is a multiplier of 2 and is greater than M, and the unit 1006 performs IFFt (Inverse Fast Fourier Transform) on the N points, and the unit 1006 performs N/M oversampling in order to obtain transmission signals Sdout(M×v+1) to Sdout(M×v+M).
PAPR after N/M-times oversampling can thus be reduced in a manner that signals which may cause high peaks after N/M-times oversampling are selected by performing 2-times oversampling and are further attenuated before DFT in N/M-times oversampling.
Non-Patent Document 1: PA power de-rating reduction scheme for DFT-SOFDM and TP, R1-060392, Motorola, 3GPP TSG-RAN WG1 #44, Denver, USA, Feb. 13-17, 2006